1. Field of the Invention
The invention relates to a process for preparing organopolysiloxanes having triorganosiloxy groups, especially linear organopolysiloxanes functionalized in a comb-like manner with triorganosiloxy groups, particularly with vinyldimethylsiloxy groups.
2. Background Art
As is commonly known, organosiloxane copolymers can be prepared in the simplest case by a condensation reaction between two reactants containing silanol groups. However, the reaction profile is not specific, since the homopolymerization of the reactants with themselves also takes place in parallel to the desired heterocondensation process. As a result, a random mixture of the desired polymer and the homopolymerization products is therefore obtained. The classical condensation process is therefore unsuitable for selectively forming linear organopolysiloxanes functionalized in a comb-like manner with organosiloxy groups.
The same applies for the introduction of T units via equilibration processes, since the distribution of the T units is also purely random. The reaction products are therefore a mixture of more or less highly branched and partly bridged polymers rather than a polymer with a defined uniform structure.
In the literature, a multitude of so-called “nonhydrolytic” reactions have therefore been proposed, with which selective formation of Si—O—Si bonds is possible. Among the functionalization routes detailed, however, for various reasons such as raw material availability, reaction selectivity, reaction yield, reaction time or specific plant requirements, only a few are suitable for the preparation of the target compounds on the industrial scale.
One example is the acid- or alkali-catalyzed dehydrocondensation between SiH-containing organopolysiloxanes and SiOH-containing organosilicon compounds. However, the reaction is often accompanied to a not inconsiderable degree, by equilibrations and the partial fragmentation of the polymer backbone of the reactant, so that the resulting products are nonuniform with regard to their structure. In addition, the restricted stability and industrial availability of the silanols required for the synthesis of the abovementioned target compounds constitute an additional disadvantage, so that the analogous transition metal-catalyzed dehydrocondensation based on noble metal catalysts is also unable to offer any industrially satisfactory alternative. In the latter process, an additional disadvantage is that the noble metal catalysts used, because they are recoverable only with a considerable degree of cost and inconvenience, if at all, remain in the product, which can lead to undesired effects in the further processing or use of the products.
A possible alternative is offered by the approach taught by Deforth and Mignani in US published application 2003/0139287. Here, boron-catalyzed crosslinking of SiH-containing organopolysiloxanes with silanol-terminated polydimethylsiloxanes with elimination of hydrogen leads to silicone elastomers. In principle, the process should also be applicable to the synthesis of the target compounds specified at the outset. The preferred boron catalysts, especially tris(pentafluorophenyl)boron, have, however not yet become industrially widespread and are accordingly expensive. Moreover, these catalysts also remain in the reaction product, which is undesired for the reasons previously mentioned.
A similar route is described by Rubinsztajn and Cella in U.S. published application 2004/0127668 and Polymer Preprints 2004, 45(1), p. 635. Here, SiH-containing silanes or siloxanes are reacted with one another under Lewis acid catalysis with elimination of a volatile hydrocarbon radical and simultaneous formation of an Si—O—Si bond. However, the process has the disadvantage that the catalysts used, for example FeCl3, AlCl3, ZnCl2, ZnBr2 or BF3, are either insoluble in siloxanes and thus have a low activity, or else, for example the tris(pentafluorophenyl)-boron used with preference, are very expensive and likewise remain in the reaction product at the end of the reaction.